The present invention relates in general to integrated circuit design and, more particularly, to reducing cross-talk between circuits disposed on a common integrated circuit substrate.
Electronic circuits commonly use digital and analog functions in the same design. In the past, the analog functions were typically placed on one integrated circuit (IC) die while the digital functions were placed on a second IC die. With ICs becoming highly compact and miniaturized, say in the 0.5 micron range, many functions may be integrated onto a single IC die. In order to save space and reduce overall costs a common trend is to integrate the digital and analog functions onto the same IC substrate.
One significant problem associated with the digital and analog integration to a common IC die is the associated cross-talk between the digital and analog circuitry. The digital circuitry operating at say 100.0 megahertz with sharp edges of say one nanosecond rise and fall times creates significant dv/dt induced voltage spikes in the substrate of the IC. These voltage spikes travel through the conductive substrate and interfere with the analog circuitry located in other areas of the IC substrate. The voltage spikes can cause variation in the power supply and ground busses as well as interfering with input signals to the analog circuitry. Basically the voltage spikes induced by digital switching interferes with the fundamental operation of the analog circuitry.
One known solution to help minimize the digital cross-talk is to place guard rings around the digital circuit and the analog circuit. For example in a p-type substrate a heavily doped p+ region surrounds the digital circuit and is connected to ground potential. The heavily doped region tends to absorb the cross-talk emanating from the digital circuit. Guard rings are effective only up to a certain frequency of operation of the digital circuit. To increase the effectiveness of the guard rings it is necessary to widen the p+ region. It is impractical in many applications to dedicate large areas of the IC die to guard ring protection.
Hence, a need exists to reduce cross-talk between digital and analog circuits that are disposed on a common IC substrate.